To cleanup Kirby https://getkirby.com/docs/cookbook/extensions/content-file-cleanup

cleanup.php

<?php

/**
 * Script to clean up unused fields in page or file content files
 * Compares fields present in the content file 
 * with fields defined in the blueprint
 * and removes all undefined fields
 */

require __DIR__ . '/kirby/bootstrap.php';

$kirby = new Kirby;

// Authenticate as almighty
$kirby->impersonate('kirby');

// Define your collection
// Don't use `$site->index()` for thousands of pages
$collection = $kirby->site()->index();

// set the fields to be ignored
$ignore = ['title', 'slug', 'template', 'sort'];

// call the script for all languages if multilang
if ($kirby->multilang() === true) {
    $languages = $kirby->languages();
    foreach ($languages as $language) {
        cleanUp($collection, $ignore, $language->code());
    }
} else {
    cleanUp($collection, $ignore);
}


function cleanUp($collection, $ignore = null, string $lang = null) {
    foreach($collection as $item) {
        // get all fields in the content file
        $contentFields = $item->content($lang)->fields();

        // unset all fields in the `$ignore` array
        foreach ($ignore as $field) {
            if (array_key_exists($field, $contentFields) === true) {
                unset($contentFields[$field]);
            }
        }

        // get the keys
        $contentFields = array_keys($contentFields);

        // get all field keys from blueprint
        $blueprintFields = array_keys($item->blueprint()->fields());

        // get all field keys that are in $contentFields but not in $blueprintFields
        $fieldsToBeDeleted = array_diff($contentFields, $blueprintFields);

        // update page only if there are any fields to be deleted
        if (count($fieldsToBeDeleted) > 0) {

            // flip keys and values and set new values to null
            $data = array_map(function($value) {
                return null;
            }, array_flip($fieldsToBeDeleted));

            // try to update the page with the data
            try {
                $item->update($data, $lang);
                echo Html::tag('p', 'The content file for ' . $item->id() . ' was updated');
            } catch (Exception $e) {
                echo Html::tag('p', $item->id() . ': ' .$e->getMessage());
            }
        } else {
            echo Html::tag('p', 'Nothing to clean up in ' . $item->id());
        }
    }
}